1. Field of the Invention
The invention relates to a drive force distribution control apparatus.
2. Discussion of Background
Examples of drive force distribution control apparatuses include a drive force distribution apparatus for a drive force transmission device of a four-wheel-drive vehicle. The drive force distribution control apparatus for a four-wheel-drive vehicle includes a clutch mechanism, and executes on-off control on a relay to control power distribution to an electromagnetic coil for engaging or disengaging the clutch mechanism.
The above-described drive force distribution control apparatus executes on-off control on switching means, such as a switch transistor, with the relay closed, thereby exciting or de-exciting the electromagnetic coil, which is electrically connected to a power line via the relay. When the electromagnetic coil is excited, the clutch mechanism is engaged so that torque distribution for four-wheel drive takes place.
In such a drive force distribution control apparatus, when an ignition switch (IG, power switch) is turned on, a process for detecting an abnormality of the relay is executed. The abnormalities to be detected through this process include abnormalities that cause the contacts in the relay to be stuck on each other, such as contact welding (i.e., abnormalities that keep the relay stuck closed) and abnormalities that prevent the contacts in the relay from contacting each other (i.e., abnormalities that keep the relay stuck open).
In the relay abnormality detection process described above, the relay is turned on and off several times while the ignition switch IG is on, and the voltage at a circuit connected to the relay is compared with a threshold voltage. Thus, whether there is an abnormality that keeps the relay stuck open is determined. When such an abnormality is detected as a result of turning the relay on and off several times, a failsafe process is immediately executed. For example, US2003-0080746 describes a technique in which, when an abnormality that keeps a relay stuck open is detected as a result of turning the relay on and off, the relay is immediately prohibited from being closed so that the drive mode is switched from the four-wheel-drive mode (four-wheel-drive control) to the two-wheel-drive mode (two-wheel-drive control).
However, according to this technique, the following inconvenience may occur. The voltage at the circuit connected to the relay is compared with the threshold voltage. Therefore, if the relay is turned on and off several times when the battery voltage is temporarily low due to a decrease in the ambient temperature, it may be erroneously determined that there is an abnormality that keeps the realty stuck open. In such a case, despite the fact that the detection of the abnormality is not correct, the process for prohibiting the relay from being closed is immediately executed to switch the drive mode from the four-wheel-drive mode to the two-wheel-drive mode, and the two-wheel-drive mode is thereafter maintained, that is, the drive mode is not switched back to the four-wheel-drive mode even when the battery voltage later returns to a normal level due to an increase in the ambient temperature. This may result in reduction in the stability of traveling (driving) of the vehicle depending upon the road surface condition.